1. Field of the Invention
The present invention relates to a thermal excitation-type sound wave generator and a method for producing the same, and to a method for generating sound waves using the sound wave generator.
2. Description of Related Art
Conventionally, various types of sound wave generators are known. Except for a few of special sound wave generators, most of the types generate sound waves by converting mechanical vibration in their vibrating part into vibration in a medium (for example, air). However, in such a sound wave generator that uses mechanical vibration, the vibrating part has a characteristic resonance frequency, and therefore the frequency bandwidth of the sound waves to be generated is narrow. In addition to this, the resonance frequency varies depending on the size of the vibrating part. Thus, miniaturization and array alignment of the generator are difficult to achieve with its frequency properties being maintained.
On the other hand, there is proposed a sound wave generator that is based on a new principle and does not use mechanical vibration. This sound wave generator is called a thermal induction-type sound wave generator and is disclosed in each of the following literatures. Nature, vol. 400, pp. 853-855, 26 Aug. 1999, discloses a sound wave generator in which a base layer (p-type crystalline Si layer) with a relatively high thermal conductivity and a heat-insulating layer (microporous Si layer) with a relatively low thermal conductivity are combined, and an Al(aluminium) thin film is further disposed thereon with the heat-insulating layer interposed between the Al thin film and the base layer. The Society of Chemical Engineers, Japan, the 37th Annual Meeting in Autumn, symposium on <nanoprocessing>, proceedings D-307 (2005), discloses a sound wave generator in which a base layer (single-crystal Si layer) with a relatively high thermal conductivity and a heat-insulating layer (nanocrystalline porous Si layer) with a relatively low thermal conductivity are combined, and a W (tungsten) thin film is further disposed thereon with the heat-insulating layer interposed between the W thin film and the base layer. Nature, vol. 400, pp. 853-855, 26 Aug. 1999, and The Society of Chemical Engineers, Japan, the 37th Annual Meeting in Autumn, symposium on <nanoprocessing>, proceedings D-307 (2005), describe that: upon the supply of electric power including an alternating current component to the Al thin film or W thin film, the temperature of the corresponding thin film periodically changes due to Joule heat; the periodic temperature change is transferred to the air in contact with the thin film without escaping to the side of the base layer because the heat-insulating layer has a low thermal conductivity; and the periodic temperature change that has been transferred to the air induces a periodical change in the density of the air so as to allow sound waves to be generated.
A thermal induction-type sound wave generator can generate sound waves without mechanical vibration. Therefore, the frequency bandwidth of the sound waves to be generated is broad. In addition to this, miniaturization and array alignment of the generator are comparatively easy to achieve.
JP 3798302 B2 discloses that heat application using a pulse current is preferable for increasing power of the sound waves to be generated, in a thermal excitation-type sound wave generator. JP 3798302 B2 further discloses a heat-insulating layer having a surface with a projection.
JP 2005-150797 A discloses a technique for applying a current that has been produced by superimposition of a direct current on an alternating current to a thermal excitation-type sound wave generator. JP 2005-150797 A describes a sound wave generator including a base layer that is a single-crystal Si substrate and a heat-insulating layer that is a porous Si layer.
JP 3845077 B2 discloses a sound wave generator including a heat-insulating layer (nanocrystalline Si layer) obtained by anodization and supercritical drying. JP 3845077 B2 further discloses that: the sound pressure to be output increases as the ratio of the thermophysical parameter αC (α: thermal conductivity, C: heat capacity) of the heat-insulating layer with respect to the αC of the base layer decreases; the αC of the heat-insulating layer decreases as the porosity of the heat-insulating layer increases; and a nanocrystalline Si layer with a porosity of 75% or more is preferable as the heat-insulating layer.
JP 3808493 B2 discloses a sound wave generator in which the ratio α1C1/αsCs (I: heat-insulating layer, S: base layer) of the αC of the heat-insulating layer with respect to the αC of the base layer satisfies the formula: 1/100≧α1C1/αsCs, and the αC of the base layer satisfies the formula: αsCs≧100×106. The technique of JP 3808493 B2 is based on a technical idea of combining a base layer and a heat-insulating layer so that the thermal contrast between the base layer and the heat-insulating layer, which is given by the formula: α1C1/αsCs, exceeds 1:100, and on a technical idea of selecting the base layer with a high αC. JP 3808493 B2 describes silicon, copper and SiO2 as a material for constituting the base layer and describes porous silicon, polyimide, SiO2, Al2O3 and polystyrene foam as a material for constituting the heat-insulating layer. The combination of the base layer composed of silicon and the heat-insulating layer composed of porous silicon is mentioned in JP 3808493 B2 as the most preferable combination of the base layer and the heat-insulating layer.